All horses have internal parasites. Ninety percent of all the colic cases may
be related to blood vessel damage caused by the migrating larvae of Strongylus
vulgaris (blood worms). Fifty percent of the deaths in horses may be related
to internal parasites.

Internal parasites have adapted themselves to the internal environment of
their host animal and have become host-specific. Horse parasites can only exist
in horses and cattle parasites only in cattle. If a cow eats the eggs or larvae
of a horse parasite (or vice versa), the life cycle of the parasite is broken.
This can be a factor in developing parasite control programs.

There are five major kinds of equine internal parasites: large and small
strongyles, ascarids, bots, and pinworms.

Large and Small Strongyles

The most common and most destructive of all internal parasites in the equine
are the large strongyles (blood worms). Strongyles can be seen in horses of all
ages, except in very young foals. Strongyles range in length from 1/2" to
2".

The sexually mature strongyles are found, for the most part, in the large
intestine. The larval stages travel via the blood vessels to the heart, liver,
and lungs, leaving damaging tracks or scar tissue behind by destroying healthy
tissue. The mature female lays her eggs which are passed in the manure. The eggs
hatch into infective larvae that contaminate grass and hay, and when ingested,
the cycle starts again. When horses are kept on the same pasture year after
year, the number of infective larvae accumulate and multiply.

The large strongyle group contains three main species: Strongylus vulgaris,
Strongylus equinus, and Strongylus edentatus. Strongylus
vulgaris, the most destructive, migrates via the arteries of the intestine,
creating a thickening in the wall of the anterior mesenteric artery where it
attaches to the aorta. The resultant thrombi (blood clot in bloodstream) or
aneurysm (ballooning of vessel wall) create a significant reduction in blood
flow to a large portion of the bowel. Periodic bouts of colic shortly after
feeding or exercise are common due to showering or movement of thrombi into
small vessels of the intestines. Severity of the colic episodes depends on the
amount of circulatory damage. Total blockage or rupture of an aneurysm can
result in death of the horse.

There are about 40 species of small strongyle (cyathostome) and they are
increasing in numbers due to drug resistance. In many areas, the small strongyle
may be more of a problem than the large strongyle. Migration is limited to the
intestinal lining. Numerous encysted small strongyles emerging from the
intestinal wall can induce colic.

The life cycle of strongyles is continuous; therefore, horses may need to be
dewormed every 8 to 10 weeks year round to control these blood sucking
parasites.

Ascarids

Parascaris equorum (large roundworms) are the largest internal
parasite affecting horses, ranging in length from 5" to 15". Ascarids
reach up to 1/2" in diameter when mature and look much like a white
earthworm. These parasites are common in young horses and are usually not found
in horses more than five years old. Immunity normally develops following
exposure to these large roundworms during adolescence.

The female roundworm deposits eggs in the intestine and the eggs pass out in
the manure. The larvae develop to infective stage within the eggs, which are
swallowed by the horse with contaminated feed or water. The eggs hatch in the
intestine, releasing larvae that penetrate the intestinal gut wall. They migrate
through the liver, then through the lungs, and then to the alveoli where they
are coughed up to the windpipe to the pharynx where they are reswallowed. The
roundworms then develop to maturity in the small intestine and the cycle repeats
itself. The life cycle is completed in 10 to 13 weeks. Respiratory problems in
young horses can be started and intensified due to larval migration of
roundworms. Worm impactions of this worm species can be seen in heavily infested
young horses not on a routine worming program.

Bots

Bots come in three types: Gastrophilus intestinalis, the most common; Gastrophilus
haemorrhoidalis, the nose bot; and Gastrophilus nasalis, the throat
bot.

The buzzing of the hairy, brown bot fly is very annoying to the horse and
owner. It is about the size of a honey bee, does not bite but lays yellow eggs
on the hair of the forelegs, shoulders, and flanks of the horse. These eggs are
stimulated, hatched, and ingested by the horse when they try to lick off the
eggs.

Larvae of all three types of bots enter the mouth and embed themselves in the
mucous lining of the gums. After about a month, they migrate to the stomach and
become attached, leaving the stomach lining inflamed and ulcerated. After 8 to
10 months of development, the larvae pass out in the manure. Once outside, the
bot hatches into an adult fly, and the cycle begins again. The bot fly
population is most active in the late summer and fall.

Bot flies do not feed. Control is directed at egg removal from the hair and a
dewormer to kill the bots in the stomach. One month after a killing frost which
will kill the remaining eggs on the hair, a boticide dewormer is administered.

Pinworms

Oxyuris equi (pinworms) are found in the cecum, colon, large
intestine, and rectum. Female worms are normally full of eggs which pass out in
the feces. Additional female pinworms crawl out the anal opening and deposit
their eggs on the surrounding skin, causing irritation and itching to the host.
For relief, the horse usually rubs against a fence post or other solid object,
resulting in hair loss near and on the tail and occasionally producing a
secondary infection.

The mature female has a slate-grey or brownish color and a narrow tail which
may be more than three times as long as the rest of the body. Females are
3" to 6" in length. The life cycle is completed in five months.

Pinworms are actually less damaging to the equine system than any other
internal parasite group. Yet the constant annoyance and irritation can spoil the
looks and action of an otherwise attractive horse. Pinworms can sometimes be
seen in the manure of heavily infected animals.

Working dirt lots with a
harrow, springtooth harrow, or disk to bury manure and destroy weed growth.

Periodical grazing of cattle
in horse pastures decreases exposure as equine parasites do not mature in
cattle and breaks the life cycle.

Treatment programs consist of:

Proper drugs at the proper
dosage and proper time (Figure 1).

Deworming all animals in the
group or lot.

Deworming pregnant mares
within 30 days of foaling to minimize passage to foal.

Keeping lactating mares and
their foals on a rigid treatment schedule (every 6 to 8 weeks).

Rotating anthelmintic classes
of drugs to prevent development of parasite resistance. Resistance generally
develops with repeated usage over time (Figure 1).

Number of horses, space allotted per horse, age of horses, contact between
horses, type of feed (i.e. pasture or dry lot) all have a bearing on frequency
of deworming. Your veterinarian should evaluate each individual circumstance and
determine the proper drug and frequency needed. Veterinarians can also provide
fecal examinations 7 to 10 days post treatment to evaluate efficacy of a control
program.

Figure
1. Three Classes of the Most Commonly Used Wormers

I.
BENZIMIDAZOLES

Brand Name

Active Ingredient

Distributor

Worms Affected

Cutter Paste Wormer

febantel

Cutter Animal Health

ascarids
strongyles
pinworms

Panacur

fenbendazole

Hoechst

ascarids
strongyles
pinworms

Safe-Guard

fenbendazole

Hoechst

ascarids
strongyles
pinworms

Telmin

mebendazole

Pitman-Moore

ascarids
strongyles
pinworms

Benzelmin

oxfendazole

Syntex

ascarids
strongyles
pinworms

Anthelcide EQ

oxibendazole

SmithKline Beecham

ascarids
strongyles
pinworms

Equipar

oxibendazole

Cooper

ascarids
strongyles
pinworms

Equizole

thiabendazole

Merck

ascarids
strongyles
pinworms

II.
TETRAHYDROPYRIMIDINES

Strongid T

pyrantel

Pfizer

ascarids
strongyles
pinworms

Strongid P

pyrantel

Pfizer

ascarids
strongyles
pinworms

Strongid C
(for continuous feeding)

pyrantel

Pfizer

ascarids
strongyles
pinworms

III.
AVERMECTINS

Eqvalan

ivermectin

Merck

bots
ascarids
strongyles
pinworms

Zimecterin

ivermectin

Farnam

bots
ascarids
strongyles

Issued in furtherance of
Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with
the U.S. Department of Agriculture. Kenneth R. Bolen, Director of Cooperative
Extension, University of Nebraska, Institute of